I'm testing this with GNU/Linux but I'm pretty sure that this is reproducible on any OS.

I thought I used to control-click a single file to toggle its selection, even when there were other files selected. But now I can't do that.

What you need to reproduce this bug: a folder with multiple EasyTAG-modifiable files (I used a folder of a few dozen FLAC files but the number of files doesn't really matter so long as you have at least 2)

Steps to reproduce the bug:
1. Open EasyTAG on the folder of files and select some of the files in any way you wish.
2. Control-click on a file to toggle its inclusion in the set of selected files. Depending on which file you control-clicked one of two things will happen:

Control-clicking an unselected file: will add that file to the selection (this is correct behavior).
Control-clicking a selected file: will not remove that file from the selection (this strikes me as incorrect behavior).

I expected control-clicking to always toggle the control-clicked file's selection status regardless of what else was in the selection set. To see the behavior I expected, unselect all selected files and control-click a single file.